1. Field of Invention
The present invention pertains to a printed wiring board used for a wiring component of a circuit configuration of an electronic apparatus. In particular, the invention relates to an improvement of a printed wiring board having an impedance matching the characteristic impedance of the transmission line.
2. Description of Related Art
Pursuant to an increase in speed of electronic circuits, the frequency of the transmitted signals has also increased, and matching of the characteristic impedance of the transmission line has become important.
Here, when the characteristic impedance of the transmission line is matched, it is effective in preventing erroneous recognition of signals and electromagnetic wave interference due to ringing, reflection, and the like. Also, even when analog signals are transmitted, it is effective in transmission of good-quality signals, having controlled the distortion, corruption, attenuation, and the like, of waveforms.
Past methods of matching the characteristic impedance of the transmission line are largely divided into those performed in stages for manufacturing printed wiring boards and those performed after having manufactured the printed wiring board. There are known various methods such as the following representative methods.
The first through third methods are performed when manufacturing printed wiring boards, and the fourth and fifth methods are performed after having manufactured the printed wiring boards.
The first method is a method that changes the width and thickness of the conductive pattern of the transmission line, and it matches the characteristic impedance of the transmission line by changing the thickness of the copper foil of the transmission line, for example, when connecting between hard printed wiring boards with a flexible printed wiring board (e.g., Japanese Laid-Open Patent No. 7-106766).
The second method adjusts the thickness of the insulating layer to meet the characteristics of the transmission line (e.g., Japanese Examined Patent No. 7-54875).
The third method adjusts the inductance of the insulating layer by selecting the material to meet the characteristics of the transmission line (e.g., Japanese Examined Patent No. 7-83185).
The fourth method provides a plurality of ground wiring layer patterns and through-holes corresponding respectively, so as to match the characteristic impedance of the transmission line after manufacturing a multilayer printed wiring board by selectively connecting the through-holes (e.g., Japanese Laid-Open Patent No. 6-252563).
The fifth method inserts a damping resistor and terminal resistor in the signal line, which generally can be said to be already well-known as a method of matching the characteristic impedance of the transmission line on a circuit.
However, in the first method (adjusting the width and thickness of the conductive pattern of the transmission line), because the width of the conductive pattern of the transmission line must be made smaller and the thickness must be made thinner, there occur technical problems when trying to increase the characteristic impedance of the transmission line. For example, the conductive pattern easily breaks, or it easily receives a direct influence of the precision of etching processing, and, due to deviation of the etching processing and structure, the fabricated transmission line simply tends to deviate from the intended characteristic impedance, or the conductive resistance ends up being increased, and the transmitted signals become attenuated needlessly.
Also, the second method (adjusting the thickness of the insulating layer), when trying to increase the characteristic impedance of the transmission line, because the thickness of the insulating layer must be made thicker, is difficult to apply to a flexible printed wiring board.
Furthermore, in the third method (adjusting the inductance of the insulating layer), material meeting the characteristics of the transmission line must be selected, and the cost simply soars, or it is difficult to accurately match the characteristic impedance of the transmission line only by inductance of the material.
Further still, since all of the first through third methods match the characteristic impedance during manufacturing of the printed wiring board, they do not permit adjustment of the characteristic impedance after manufacturing.
Also, in the fourth method (selectively connecting the ground wiring layers), there are technical problems because a plurality of ground wiring layers and a plurality of through-holes must first be established so that the through-holes can be selectively connected for adjusting the characteristic impedance. For example, space for the ground wiring layers and through-holes becomes unavailable for adjusting the characteristic impedance, and the printed wiring board simply becomes larger in scale by that amount, or the number of layers of the printed wiring board tends to increase, and it becomes difficult to apply to a flexible printed wiring board in that condition.
Furthermore, in the fifth method (inserting a damping resistor and a terminal resistor), for the generally well-known method of inserting a damping resistor, since the time constant of the circuit consequent to the damping resistor becomes greater, there are technical problems. For example, the potential of the arriving signals is decreased, and decrease of transmission speed is caused by corruption of the waveforms. On the other hand, in the method of inserting a terminal resistor, since the power consumption is increased by the amount of terminal resistors inserted, the drive power must be increased, and the power capacity is increased by that amount, thus a condition is caused that does not meet the demand for reduction of power cost.